In the manufacture of steel, furnaces are often lined with refractory material. This refractory material wears as a result of the process. After the refractory material has worn down to predetermined levels it must be replaced, otherwise a dangerous situation may arise wherein the molten steel will leak from the furnace. Steel making equipment is quite expensive and the economic efficiency of such equipment is dependent upon the length of time a furnace can be employed without having to be shut down for the providing of a new refractory lining. Therefore, it is of substantial economic significance to employ a refractory lining as long as possible, yet it is also important not to allow the lining to be used when it has worn below a safe minimum thickness.
Presently, several methods have been employed for monitoring the thickness of refractory linings in steel making equipment. One of the best is described in U.S. Pat. No. 4,025,192 by the same applicant and invented by Ragnar Scholdstrom et al. In accordance with that method measuring is made to three non-collinear reference points on the object from a distance measuring instrument placed in an arbitrary position in front of a furnace. This measuring is made with the furnace already in a measuring position with its front surface turned to the distance measuring device. In order to make sure that the reference points can be seen from the distance measuring device these points most often have to be placed on the front side of the furnace surrounding the opening to the inside of the furnace. There is a tendency for a deposit of slag to form on the front side of the furnace during the oxygen blowing process, when the molten steel is poured out from the furnace through said opening and also when, during the melting process, materials are transported through the opening. This deposit can often amount to a thickness of several decimeters. Therefore, in order to perform this known method the front side of the furnace must be cleaned now and then in order to uncover the marks on the reference points.